The purpose of the proposed research is to elucidate several aspects of the formation and function of C3e, a recently identified leukocytosis promoting agent derived from the third component of complaint. I will identify, purify, and characterize the plasma enzyme(s) responsible for its formation from C3c and ascertain whether any of the proteases of the blood-clotting system in general and the Hageman factor-dependent pathways in particular might act to digest C3c. (activated Hageman factor (HFa), Hageman factor fragment (HFf), kallikrein, plasmin, and factor XIa]. Since C3e causes leukocytosis and neutrophil infiltration upon intradermal injection but does not possess chemotactic activity, its interaction with neutrophils will be characterized and the ability of decomplemented rabbits to mediate C3e-dependent cellular infiltration will be examined. Neutrophils (and other cells) will be tested for their ability to bind C3e and evidence of a bone-fide C3e receptor on such cells will be sought. The ability of C3e to stimulate such cells to release chemotactic factors or to release enzymes which can digest C5 to yield C5a chemotactic factor will be tested. The structural features of C3e which might be required for its activity upon neutrophils will be examined including a determination of its amino-and carbosy-terminal residues, its susceptability to inactivation by enzymatic digestion, and its structural features as assessed by circular dichroism. Antisera will be prepared to C3e and absorbed with native C3 and other C3 fragments in order to identify a neoantigen in C3e; using such antisera, a radioimmunoassay for C3e will be developed. Finally, the information regarding C3e function will be applied to studies of cellular release from long bones in order to define these parameter of C3e function that are needed in order to stimulate a peripheral blood leukocytosis. In order to fulfill these studies, sufficient amounts of homogenous preparations of C3e will be prepared, and have the necessary know-how to achieve such a goal.